The metabolism of the rates of N-dealkylation of methsuximide and phensuximide in epileptic patients were determined. Methsuximide 1200 mg and Phensuximide 1200 mg were administered to an epileptic patient at different times. The plasma levels of each drug and their N-dealkylated metabolites were determined by mass fragmentography. The t 1/2 of phensuximide following oral administration was 7 1/2 hours. The t 1/2 of the N-desmethyl phensuximide was similar to that of the parent compound. However, the plasma levels were only 1/3 that of phensuximide. The t 1/2 of methsuximide was 1 1/2 hours and that of the N-desmethylmethsuximide was 28 hours. It would appear from these results that the antiepileptic activity of methsuximide was due to N-desmethylmethsuximide whereas the activity of phensuximide was due to both parent and N-dealkylated metabolite.